1. Field of Technology
The present disclosure relates to a system for optical, open space data communication by means of digital signals between an optical transmitting device and an optical receiver utilizing optical radiation modulated by an array of emitters, which emit radiation at multiple levels of varying radiation output, wherein the information carrier is formed by the output of the emitted radiation.
2. Brief Discussion of Related Art
Modern mobile data processing systems require high data transmission bandwidths for such applications as the Internet or for high-quality audio and/or video streaming. Currently, data is transmitted predominantly in the radio frequency band (RF); however, the increase in number of such data transmission systems is leading to increased exhaustion of the available frequency bands. For this reason, efforts are being made to implement such data transmission using optical open space data communication systems. In these systems data is transmitted between an optical transmitter and an optical receiver by modulating the radiation output (or optical power output or light intensity) emitted by the transmitter. During transmission a primarily direct visual contact between the transmitter and the receiver is utilized. However, it is also possible to carry out an optical transmission using diffused light. A further advantage of such optical open space data communication systems is that it is possible to comply with strict requirements relating to electromagnetic compatibility, such as, for example, in hospitals or in sensitive industrial facilities.
The simplest type of optical data transmission is binary data transmission via a pulsed radiation source, that is, the switching on and off of the radiation source or operating with two different light intensities. However, only a limited data rate is achievable with this type of data transmission.
In contrast, the so-called orthogonal frequency division multiplexing method (OFDM) is a suitable method of transmission with which it is possible to achieve high transmission rates, even in situations involving multi-path reception. A major disadvantage of this method is that it necessitates employing complex transmitter devices (whether in the radio or optical range) with very high linearity and a large dynamic range. For transmitting optically in open space, laser diodes or LEDs in the visible or infrared spectrum are generally used, because these alone are fast enough to convert an electrical current modulated with multiple MHz to a modulated radiation output. The main disadvantage of LEDs is that the relation between emitted radiation output and electrical current is to a high degree non-linear. The resulting non-linearities are problematic, especially in conjunction with the OFDM method, and they result in poor signal/noise ratios (SNR) or poor signal/interference plus noise ratios (SINR) and, accordingly, in reduced transmission quality, if complex and costly technical measures for purposes of linearization are not undertaken. Moreover, such traditional transmitting devices require digital-to-analog converters and transconductance amplifiers which exhibit low energy efficiency at high frequencies and bandwidths.
An arrangement of the aforementioned kind is known from DE 102007043255 A1 in which as many radiation emitters are present as are discrete values to be transmitted. For example, there are 255 emitters in the transmission of an 8-bit signal.
Systems for optical data communication having a transmitting device as well as an optical receiver are known from U.S. 2009/0297167 A1, U.S. 2002/0126338, U.S. 6775480 B1 and WO 2008/001262 A1, in which the transmitting device comprises a modulator for modifying the emitted radiation.